Originally posted by Spock +1
I had a A501.4 and while it was a lovely lens after not using it for about a year (too long on the digital) I sold it.
I use my 50 1.4 mostly for long exposures at night--just gorgeous colors.
It is also decent for indoor low light if you can get back far enough.
I lot of historical sites/museums will not let you use a flash indoors--hence the need for a good low light lens. For instance, we are going to visit York Minster--I will probably not be able to use a flash and will need to handhold to shoot.